For an Allen wrench, it mainly depends on the size of the corresponding screw. Common specifications are: 2, 2.5, 3, 4, 5, 6, 7, 8, 10, 12, 14, 17, 19, 22, 24.
For external hexagon wrench, it mainly corresponds to the size of bolt or nut. Its specifications range from 5.57 to 36 (unit: mm), and the common ones are M3, M4, m 5, M6, M8, M 10, M 12, M 14, M 16 and M/kloc-.